Method for producing hydrogen

ABSTRACT

The invention relates to a method for producing hydrogen by reacting amorphous silicon with water, an alcohol or a carboxylic acid. Said method can be carried out independently of carbon sources and water sources and performed in situ without any hydrogen transport or storage problems.

The present invention is directed to a method for producing hydrogen.

Hydrogen is conventionally produced from hydrocarbons, i.e. fromhydrocarbons containing energy carriers coal, mineral oil, natural gas.Furthermore, it is known to obtain electrolytically produced hydrogenfrom water. However, this method is very energy consuming (about 5kWh/m³H₂) Moreover, water is available to a great extent only in certainregions of the world (not in desert regions). If one takes into accountthe diffusion behaviour of hydrogen, its storage and its transport arevery dangerous since explosive mixtures (oxyhydrogen gas) are generatedupon a mixing with air. A hydrogen liquefaction for storage purposes isconnected with a high energy expense.

Hydrogen is considered as energy source of the future since with thegeneration of energy from hydrogen (combustion with oxygen for thegeneration of water) no gases (CO, CO₂, SO₂ etc.) which are detrimentalfor the environment are generated. However, on the other side theconventional production of hydrogen from hydrocarbons results in thegeneration of substances (CO, CO₂ etc.) which are detrimental for theenvironment and which are just to be avoided when energy is generatedfrom hydrogen. Accordingly, in the last analysis this way for theproduction of hydrogen is no solution for the growing environmentalproblems and moreover results in an enforced exploitation of thecoal/gas/oil reserves. In the last analysis, the problem ofenvironmental pollution is only shifted from the place of energygeneration to the place of hydrogen generation by this conventionalproduction of hydrogen.

Accordingly, a method for producing hydrogen is desirable which can becarried out in situ and not from C sources.

It is the object of the invention to provide a method for the productionof hydrogen which can be carried out independently of C sources.

The invention shows three ways to achieve this object. According to afirst way of solution a method for producing hydrogen by the reaction ofamorphous silicon with water is provided.

Amorphous silicon serves as starting substance for the inventive method.The production of amorphous silicon is known and is moreover proposedwith new methods in the German patent applications 102 17 140.8, 102 17124.6 and 102 17 126.2. In the last analysis, silicon dioxide serves asstarting substance for the production of amorphous silicon. Silicondioxide is available as natural resource to a great extent in the world(especially also in desert regions) so that armorphous silicon is a safesource for the production of hydrogen with which the hydrogen can beproduced in situ, i.e. independently of C sources and/or water sources,without transport and storage problems.

Solids are designated as amorphous if their molecular components are notarranged in cristal lattices but irregularly. Amorphous silicon (a-Si)can be produced in a substantially less expensive manner thancristalline silicon.

It is expressly emphasized that the limits between amorphous andmicrocristalline or finest cristalline silicon cannot be exactly fixed.Accordingly, the invention includes that the inventive production ofhydrogen can be also carried out with microcristalline or finestcristalline silicon. Appropriate limits have to be empiricallyascertained.

In the above-cited older German patent application 102 17 140.8 it ismentioned that on the one side pure amorphous silicon having a blackcolour and not being “surface covered” and having an especially highreactivity and on the other side amorphous silicon resulting as brownpowder and being “surface covered”, for instance with Cl, silylchloride, O₂ or HO, are existing. With “surface covered” a chemicalcovering is meant.

On principle, the inventive method can be carried out with both kinds ofamorphous silicon wherein the black amorphous silicon which is notchemically covered generally has a better reactivity than the chemicallycovered brown (yellow) amorphous silicon.

Accordingly, it is preferred to use black chemically uncovered amorphoussilicon for the inventive method.

The inventive method can be preferably carried out at ambienttemperature if a corresponding reactivity of the amorphous silicon ispresent which is especially the case with the black chemically uncoveredamorphous silicon.

When using brown chemically covered amorphous silicon the reactivity ofthe silicon is dependent on the covering. So, the reactivity (reactiontemperature) of the amorphous silicon can be controlled by control ofthe chemical covering. Experiments had the result that amorphous brownsilicon covered with NH₃ has a better reactivity than amorphous brownsilicon covered with O₂.

As mentioned above, under certain circumstances the inventive method canbe carried out with microcristalline or finest cristalline silicon aspowder either wherein this substance has a still lower reactivity thanthe above-mentioned brown amorphous silicon.

In the reaction of the amorphous silicon with water for the productionof hydrogen (hydrolysis of Si_(am)) in addition to hydrogen silicondioxide (SiO₂) is generated which can be exploited or recycled.

According to the second way for carrying out the inventive methodhydrogen is produced by the reaction of amorphous silicon with analcohol. Preferably, such alcohols (ROH) are used in which R means Me(methyl) or Et (ethyl). The reaction of Si_(am) with alcohols results intetraalkoxy silanes (Si(OR)₄) wherein R generally means an organicradical, preferably alkyl radical. These tetraalkoxy silanes which arealso designated as silica esters, are conventionally produced by thereaction of silicon halides with alcohols. With the inventive method thedirect production from silicon results so that one method step is saved.There are a plurality of kinds of application for the tetraalkoxysilanes so that these compounds have a great importance as byproductsresulting from the inventive method.

According to the third way of the inventive method hydrogen is producedby the reaction of amorphous silicon with a carboxylic acid. Preferably,acetic acid (CH₃COOH) is used wherein in the reaction of Siam withacetic acid solid silicon tetraacetate is generated which has greatimportance as basic substance for establishing organo silanes andsiloxanes/silicones.

Preferably, it is moreover provided according to the invention that thecompounds Si(OR)₄, wherein R is an organic radical, especially alkylradical or carboxylic acid radical, generated during the hydrogenproduction with an alcohol or a carboxylid acid are converted intoSiO₂+HOR by hydrolysis. In this manner the alcohol or the carboxylicacid (acetic acid) can be recovered if the compound Si(OR)₄, especiallySi(OAc)₄, is no more needed.

The H₂ equimolarly produced in this manner can be used in mobile systems(fuel cell) and stationary systems. As regards the use in fuel cells, upto now the required hydrogen was produced from CH₃OH or CH₄ inconverters upstream of the fuel cell wherein in any case CO₂ wasproduced. In contrast, the inventive method is CO₂-free and valuableproducts result which are practically non-toxic and can be recycled ifthe demand is met.

As already mentioned above, with the brown amorphous silicon anytemperatures for the reaction for the production of hydrogen can beadjusted by deactivation of the Si surface (chemical covering).

In the following the reaction process of the inventive method is shownfor methyl alcohol, ethyl alcohol on the one side and for acetic acid onthe other side.

-   -   R=methyl, ethyl    -   am=amorphous    -   bl=black    -   Ac=acetate

1. A method for producing hydrogen by reacting amorphous silicon withwater.
 2. A method for producing hydrogen by reacting amorphous siliconwith an alcohol.
 3. A method for producing hydrogen by reactingamorphous silicon with a carboxylic acid.
 4. The method according toclaim 1, characterized in that black chemically uncovered amorphoussilicon is used.
 5. The method according to claim 1, characterized inthat it is carried out at ambient temperature.
 6. The method accordingto claim 1, characterized in that brown chemically covered amorphoussilicon is used.
 7. The method according to claim 6, characterized inthat the reactivity (reaction temperature) of the amorphous silicon iscontrolled by control of the chemical covering of the same.
 8. Themethod according to claim 2, characterized in that the compoundsSi(OR)₄, wherein R means an organic radical, especially alkyl radical orcarboxylic acid radical, generate( during the hydrogen production withan alcohol or with a carboxylic acid are converted into SiO₂+HOR byhydrolysis.